I had a one night stand 15 years ago and now she came back with a kid, my kids have completely 

How about not cheating? Why do cheaters always throw a pity party like you can turn back time.

So, OP is now getting the results of his actions? Nice, sad for his huge family.

42-15=27 op "i was pretty young" Me: i dont think so 😂

How many kids does this guy have?

You cannot call a single woman easy after a ONS if you are a married man 😅

Okay here is a suggestion how about ignore the other kid anyway you been doing it for 15 years. That mom can't make you do anything but pay money so who cares. Can state in court and to the girl "I've already been out of her life for so long and I'll only make things worse" in more eloquent words and at best get optional visitation and try fixing the mess you made with your family. Don't continue ruin your family to make up for a kid that literally has no other connection to you over your kids who are clearly reeling from the experience.

I mean did that ons happened before or after marriage? If before then they have right to be upset or anything but if that happened before they are married then yup op deserves it

Ummm, beyond guy is a jerk, I think the AP is trying to take the daddy.